Method of increasing the corrosion resistance of aluminum alloys and products resulting therefrom



Patented Nov. 30, 1937 UNITED STATES 2,100,411 METHOD or mcmmsRESISTANCE OF AL PATENT OFF ING THE CORROSION UMINUM ALLOYS AND PRODUCTSRESULTING THEREFROM No Drawing. Application October 15, 1935, Se-

rial No. 45,120.

4 Claims.

having a thickness of about 5% to about 7% of the thickness of theobject to be plated were employed. Since pure aluminum and thecorrosicn-resistant aluminum alloys were free from decrease the over-allstrength of the plated object. It has been proposed, for example, in U.S. Patent No. 1,927,945, to obtain the desired hard alloys for theplating which resistance to corrosion also have the same strength andhardness as the alloys constituting the base. In practice, however, thefavorable mechanical properties of the aluminum alloys were usuallyobtained by the A further object of provide a the over-all strength ofsuch articles.

The invention also contemplates providing a new article of manufacturecomprising a core of corrodible age hardening aluminum alloy having InGermany October 16, 1934 Other objects and advantages of the inventionwill become apparent from the following description. 1

It has been discovered that the corrosion resistance may be considerablyincreased by providing the base with a layer of aluminum alloysubstantially free from copper rimentally influencing the mechanicalproperties, strength or the alloy of the base by means of rolling in theconventional manner.

The thickness of this about 510 0., the thin coating iscaused topractically completely diffuse into the surface of the This subsequentheat treatment is carried out in the conventional manner with respectand time. Preferably the corresponding to the of the 'material whichrequire more than this time before the heat reaches the inside of theobtion, the conventional plating of 5% in thickness of aluminum alloyscontaining about 4% about 0.5% magnesium and about 0.5% manganesereduced the strength by about 8% with respect to the full cross section.

By proceeding according to the present P ss, the aluminum practicallycompletely difluaea copper,

. per,

into the alloy constituting the base I and the inside until it reachesthe usual values in the inchemical resistance of the surface of theobiect terior oi the article. The corrosion resistance or article isconsiderably increased compared to of the outermost surface layer orskin remains that of the unplated object or article. This will clearlyappear from the following table. Two aluminum alloys containing about4.2% copabout 0.5% magnesium and about 0.6% manganese each after a heattreatment and'in the form of sheets of about 1 mm. in thickness havebeen subjected for 30 days to the corrosion test of Myllus. Alloy I wasunplated whereas Alloy II has been provided with 1% coating of purealuminum and has been treated according invention. After the presentheat treatment, the plating 01 Alloy II practically completelydisappeared and became an integral part of the object or article. Thecomparison of the experimental values clearly demostrates the newresults obtained by the invention and the advantages thereof. 1

Alloy I Alloy II a Tensile Elonga- Tensile Elongation strong tion salamPercent kalmm Percent Initial condition 38. 5 l8. 39. 0 l8. 0 After 10days eorrosio 28. 8 2. 39. 0 15.0 After 20 days corrosion. 27. 0 2. 739. 2 l7. 0 After30daysoorrosion.. 25.0 2.6 39.0 15.5

It is to be noted that the present invention provides a method ofincreasing the corrosion resistance of aluminum alloys by plating themwith pure aluminum or aluminum alloys free from or poor in copper whichcomprises uniting a film, plate, clad, layer or coating of aluminum orsuch aluminum alloys to a base constituted of aluminum alloy of the agehardening type by means of welding without reducing strength andhardness, arranging the plating in the form of a coating of about 1% inthickness of that of the object and so thin that during the subsequentconventional heat treatment it will practically completely disappear bydiffusion and will be found to cease existing in the form of anindependent layer, and then subjecting the thus plated article to a heattreatment involving heating to a temperature of about 500 C. to about520 C. for about .to 30 minutes longer than the annealing time usuallyemployed for age hardening aluminum alloys.

It is also to be observed that the present invention provides a methodof making aluminum alloys resistant to corrosion by plating with purealuminum or with aluminum alloys free from copper characterized by thefact that the plating layer is provided in a thickness of the thicknessof the object to be protected and is completely alloyed with the metalof the base by means, of diilusion during the subsequent heat treatmentso that the plating practically ceases to exist in the form of anindependent layer and becomes incorporated as an integral part of theobject.

Furthermore, the present process possesses considerable advantages dueto the factthat the diffusion does not provide a uniform copper contentthrough the outer skin, but the copper content is dilutedin a uniformlydecreasing manner from the inside of the article to underneath the outerskin. In the outermost surface or skin, there is hardly any copper atall, but the copper content rapidly increases progressing toward theabout 1% of very good, and of course, this fact determines the corrosionresistance of the entire object. At the same time, the rapidlyincreasing copper content of the outer layer or skin permits completelyretaining the strength of the alloy constituting the inner or core.

Moreover, the present improved heat treated alloys are not to beconfused-with prior aluminum alloys such as Duralumin which has beenplated only with pure aluminum. Due to its (about 4.5% of copper, about0.5 to 1% of magnesium, about 0.5 to 1% of manganese, about 0.4 to 0.7%of silicon, and remainder aluminum). Duralumin possesses excellentmechanical strength of about 40 kg/mm, about the strength of steel, butits resistance to corrosion, The layers of plating, on the other hand,have a high resistance to corrosion but have a rather low strength. Purealuminum soft annealed, for example, has only a strength of about 8kg/mm'. The plating of Duralumin has been carried out in different ways.An example of a plating process of this character is disclosed in U. 8.Patent No. 1,865,089. Although it was possible to make Duraluminresistant to corrosion, the plating layer was rather soft.Furthermora'the plating reduced the. strength of the material to acertain extent because a plating having 5% of the cross section inthickness on each side of the object plated amounted to about 10% of thefull cross section and of the total weight. This reduction in strengthcalculated on the weight of the; object was particularly objectionablein the airplane industry which started to use aluminum alloys 'insteadof pure aluminum. For the aforesaid platings, reference is made to U. 8.Patents Nos. 1,805,448; 1,975,105, and 1,997,165. All of these prioralloys have both strength and hardness in the exterior layer but theyare incapable of reaching the strength of Duralumin. The principalreason for this shortcoming is that there is no copper in these alloys.Copper is omitted on purpose due to the well-known fact that copperreduces the corrosion-resistance of aluminum alloys. On the other hand,it is copper, especially in connection with magnesium, which makes itpossible to considerably increase the strength and hardness of the alloyby aging the alloy after heating and quenching. In consideration of thisdifficulty, it has been suggested in U. S. Patent No. 1,927,945 to platewith copper-containing alloys. This caused a small decrease in thecorrosion-resistance of the covering layer, but, on the other hand,permitted considerably increasing the strength of the complete surface.Since the covering layer has to contain less copper than the alloyconstituting the base (practically about 0.5 to 1%), it is possible toobtain higher resistance against corrosion than would be possible withan uncoatedalloy. In any event, all of these prior methods havedisadvantages and must not be confused with the present method which hasbeen described in detail hereinbefore.

What is claimed is:-

1. The process for producing a high strength integral corrosionresistant aluminum article containing an appreciable amount of copperand having a surface hardness greater than and a corrosion resistancesubstantially equal to that of pure aluminum which comprises welding avery thin layer of pure aluminum to a core of aluminumcopper alloy ofthe age-hardening type to form a composite article having (1) a highstrength, corrodible core, (2) a low strength, corrosion resistantcoating, and (3) a bond containing copper and uniting said core and saidcoating and having a thinness approximating that of a conventional bond;heat treating said composite article at an annealing temperature beyondthe usual annealing time for a period up to 30 minutes to causeaccentuated diffusion of said copper throughout said coating; stoppingsaid heating just prior to the occurrence of the copper in the surfaceskin of said coating whereby the bond and coating substantiallydisappear and an integral article is formed having high strength andhaving copper difiused substantially throughout the same except in thesurface skin which is practically devoid of copper and which has ahardness greater than that of pure aluminum and a corrosion resistancepractically that of pure aluminum.

2. The process for producing a high strength integral corrosionresistant aluminum article containing an appreciable amount of copperand having a surface hardness greater than and a corrosion resistantsubstantially equal to that of pure aluminum which comprises welding avery thin layer of pure aluminum to a core of aluminum-copper alloy ofthe age-hardening type to form a composite article having (1) a highstrength, corrodible core, (2) a low strength, corrosion resistantcoating, and (3) a bond con taining copper and uniting said core andsaid coating and having a thinness approximating that of a conventionalbond; heating said composite article at a temperature within a range ctabout 500 C. to about 520 C. for a period of time sufiicient to causedifiusion of copper throughout a zone having a thinness approximatingthat of a conventional bonding zone; continuing said heating for anextended period to cause accentuated diifusion of said copper throughoutsaid coating; stopping said heating just prior to the occurrence of thecopper in the surface skin of said coating whereby the bond and coatingsubstantially disappear and an integral article is formed having highstrength and having copper diifused substantially throughout the sameexcept in the surface skin which is greater than that of pure aluminumand a corrosion resistance practically that of pure aluminum.

3. The process for producing a high strength integral corrosionresistant aluminum article containing an appreciable amount of copperand having asurface hardness greater than and a corrosion resistancesubstantially equal to that of pure aluminum which comprises uniting acore of aluminum-copper alloy of the age-hardening type to a layer ofpure aluminum having a thickness of about 0.01th of that of said core toform a composite article having (1) a. high tensile strength, corrodiblecore, (2) a low strength, corrosion resistant coating, and (3) a bondcontaining copper and uniting said core and said very thin coating andhaving a thinness approximating that of a conventional bond, heatingsaid composite article at temperatures of about 500 C. to about 520 C.for a period of time suflicient to cause diffusion of copper throughouta zone having a thinness approximating that of a conventional bondingzone, continuing said heating for an extended period of time to causeaccentuated difiusion of said copper throughout said coating, stoppingsaid heating just prior to the occurrence of copper in the surface skinof said coating whereby the bond and coating disappear and an integralarticle is formed having high strength and having'copper diifusedsubstantially throughout the same, except in the surface skin which ispractically devoid of copper and which has a hardness greater than thatof pure aluminum and a corrosion resistance practically that of purealuminum.

4. As a new article of manufacture, a high strength corrosion resistantintegral aluminum article containing appreciable amounts of coppercomprising a core and coating having copper diffused substantiallythroughout said article, and a surface skin consisting of pure aluminumon said article practically devoid of copper, and having a hardnessgreater than and a corrosion resistance substantially equal to that ofpure aluminum, said article having no sharp line of demarcation from theinterior thereof to said surface skin and having an over all tensilestrength substantially equal to that or an agehardeningcopper-containing aluminum alloy, the copper being diifused indecreasing amounts fromthe core up to the surface skin.

OTTO REULEAUX.

